fix: preserve trailingSlash when error boundary truncates the branch

When an error occurs during loading, the client router truncates the
branch to the nearest error boundary. If trailingSlash was configured
at the page level (below the error boundary), the setting was lost,
causing the URL to be rewritten without the trailing slash.

Preserve the trailingSlash value from the full branch before truncation
by propagating it to the error node in both load_route and
set_nearest_error_page.

Fixes #13516

Author: FrankFMY

.changeset/fix-trailing-slash-error-boundary.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: preserve `trailingSlash` config when error boundary truncates the branch

packages/kit/src/runtime/client/client.js

@@ -1193,10 +1193,24 @@ async function load_route({ id, invalidating, url, params, route, preload }) {
 
 				const error_load = await load_nearest_error_page(i, branch, errors);
 				if (error_load) {
+					// Preserve trailingSlash from the full branch before truncation,
+					// so it's not lost when the error boundary is above the node
+					// that defines it (#13516)
+					let slash;
+					for (const node of branch) {
+						if (node?.slash !== undefined) slash = node.slash;
+					}
+
+					const error_branch = branch.slice(0, error_load.idx).concat(error_load.node);
+
+					if (slash !== undefined) {
+						error_load.node.slash = slash;
+					}
+
 					return get_navigation_result_from_branch({
 						url,
 						params,
-						branch: branch.slice(0, error_load.idx).concat(error_load.node),
+						branch: error_branch,
 						status,
 						error,
 						route
@@ -2401,6 +2415,16 @@ export async function set_nearest_error_page(error, status = 500) {
 
 	const error_load = await load_nearest_error_page(current.branch.length, branch, route.errors);
 	if (error_load) {
+		// Preserve trailingSlash from the full branch before truncation (#13516)
+		let slash;
+		for (const node of branch) {
+			if (node?.slash !== undefined) slash = node.slash;
+		}
+
+		if (slash !== undefined) {
+			error_load.node.slash = slash;
+		}
+
 		const navigation_result = get_navigation_result_from_branch({
 			url,
 			params: current.params,

packages/kit/test/apps/basics/src/routes/routing/trailing-slash/always/+error.svelte

@@ -0,0 +1,6 @@
+<script>
+	import { page } from '$app/state';
+</script>
+
+<p id="trailing-slash-error">{page.error?.message}</p>
+<p id="trailing-slash-error-url">{page.url.pathname}</p>

packages/kit/test/apps/basics/src/routes/routing/trailing-slash/always/+layout.svelte

@@ -0,0 +1 @@
+<slot />

packages/kit/test/apps/basics/src/routes/routing/trailing-slash/always/+page.svelte

@@ -0,0 +1 @@
+<a href="/routing/trailing-slash/always/error-boundary/">error boundary test</a>

packages/kit/test/apps/basics/src/routes/routing/trailing-slash/always/error-boundary/+page.js

@@ -0,0 +1,5 @@
+export const trailingSlash = 'always';
+
+export function load() {
+	throw new Error('trailing slash error test');
+}

packages/kit/test/apps/basics/src/routes/routing/trailing-slash/always/error-boundary/+page.svelte

@@ -0,0 +1 @@
+<p>This should not be rendered</p>

packages/kit/test/apps/basics/test/cross-platform/client.test.js

@@ -1132,6 +1132,20 @@ test.describe('Routing', () => {
 		expect(new URL(page.url()).pathname).toBe('/routing/trailing-slash/never');
 		await expect(page.locator('p')).toHaveText('/routing/trailing-slash/never');
 	});
+
+	test('trailing slash is preserved when error boundary is above page config', async ({
+		page,
+		clicknav
+	}) => {
+		await page.goto('/routing/trailing-slash/always/');
+		await clicknav('a[href="/routing/trailing-slash/always/error-boundary/"]');
+		expect(new URL(page.url()).pathname).toBe(
+			'/routing/trailing-slash/always/error-boundary/'
+		);
+		await expect(page.locator('#trailing-slash-error')).toHaveText(
+			'trailing slash error test'
+		);
+	});
 });
 
 test.describe('Shadow DOM', () => {